The present invention relates to monitoring the ground circuit in an electrical power distribution system of a type that is commonly employed at a location where coal is mined from the surface; i.e., at a strip mine. In surface mining operations, extensive use is made of large electrically powered machinery such as power shovels, pumps, drills, etc. As an example, 600 volt to 25 kilovolt, three phase electrical service may be provided at a remote surface mining site. Electrical utility company power lines are brought to a substation at or close to the mining site. Because much of the mining machinery must be movable, long insulated power cables that can be dragged along the ground connect the machinery to mobile switch houses that in turn are connected to the substation by additional long insulated power cables. There are a number of different types of power cables used in surface mining. One such type of cable includes the three power conductors, each with a metallic shield about it, two bare ground conductors, and an insulated ground check wire that commonly is called a pilot wire. All are enclosed in an insulating elastomeric jacket. Typically, the cables are in 1,000 feet lengths. Two or more cables may be connected together.
If an insulation breakdown occurs in one of the large electrically powered pieces of machinery the entire machine may be at a dangerous high voltage unless it is protected by a suitable metallic ground circuit. In many instances the machine will be in contact with the earth, but because the machinery is moved around from place to place, a dependable earth ground resulting only from contact with the earth cannot be relied upon. Consequently, a metallic ground circuit in the power cable is essential. The Federal Coal Mine Health and Safety Act of 1969 requires that the electrical ground circuit in the power cable be continuously monitored to detect short circuits, open circuits, and abnormally high resistance conditions in the ground circuit. Upon detection of a fault condition the ground monitor must open one or more circuit breakers to disconnect the electrical source from the faulty portion of the system.
A ground monitor intended for use at a surface mining location must be able to detect the desired condition that it is the metallic ground conductor of the power cable that is providing ground continuity in the system and not an earth ground resulting from the machine being in contact with the earth or water.